1. Field of the Invention
The present invention relates to an aluminum alloy brazing sheet excellent in brazability, for use in a heat exchanger of an automobile or the like.
2. Background Art
Conventionally, the side support member (reinforcing material) for use in a radiator or a heater core of a heat exchanger for an automobile, or the like is often in a two-layered structure including a core material and a brazing material (e.g., JP-A No. 11-241136 (see, paragraph 0020)). To the core material, Mg is added in an amount of about 0.2 mass %, and further, the gage is set at a prescribed or larger thickness to achieve a higher strength (see, e.g., JP-A No. 2007-131872 (see, paragraph 0010)).
On the other hand, with a recent trend toward light-weight automobiles, there has been an increasing demand for gauge down of the side support member. However, for example, when the side support member to which the technology in accordance with JP-A No. 11-241136 (see, paragraph 0020) or JP-A No. 2007-131872 (see, paragraph 0010) is applied is thinned as it is, it may be impossible to ensure the sufficient strength as a heat exchanger. Further, in order to ensure a further strength, it can also be considered that the amount of Mg to be added in the core material is increased. However, generally, it is known that Mg reduces the wettability of the brazing filler metal, and adversely affects the brazability.
Herein, the radiator of a general heat exchanger has the cross-sectional shape as illustrated in FIG. 3. Namely, a radiator 100 has a structure in which between tubes 30 for passing a refrigerant therethrough and side support members 40 for reinforcement, fins 50 are disposed, respectively, and header plates 20 are respectively attached to the opposite ends of the tubes 30 and the side support members 40. Incidentally, in FIG. 3, the length, thickness, width, and the like of each member are illustrated in an exaggerated form. Further, brazing materials 22, 32, and 42 are generally melted by heating during brazing, but are left and illustrated for convenience of description.
The side support member 40 of such a radiator 100 is generally formed by using a brazing sheet of a two-layered structure in which on one side of a core material 41, the brazing material 42 is disposed (in FIG. 3, the brazing sheet has been brazed, but the brazing material 42 is left and illustrated for convenience of description). Accordingly, the core material 41 is directly brazed to the header plate 20 at some sites. Therefore, the brazing material 22 of the header plate 20 which is another member is spread over the surface of the core material 41 by heating for brazing, resulting in formation of fillets F (portions of the brazing filler metal protruding out of the clearances of the joints).
Thus, when the core material 41 of the side support member 40 required to be brazed to other members contains Mg for ensuring the strength as described above, Mg directly reacts with the flux during braze heating, so that high-melting-point compounds such as MgF2 become more likely to be formed. Then, as a result, the oxide film breaking action due to the flux is reduced, which inhibits the wettability of the brazing filler metal. This results in reduction of the brazability.
Under such circumstances, conventionally, in order to avoid such a problem, an intermediate layer is provided between the core material and the brazing material, thereby to ensure the strength without increasing the amount of Mg added (e.g., JP-A No. 2005-015857 (see, paragraph 0022)); alternatively, on the other side of the core material (the surface opposite to the surface on which the brazing material is provided), a layer not containing Mg is provided, thereby to improve the brazability (e.g., JP-A No. 2002-273598 (see, paragraph 0020)).
However, with each method described in JP-A No. 2005-015857 (see, paragraph 0022) and JP-A No. 2002-273598 (see, paragraph 0020), other than the core material and the brazing material, another layer is provided. This unfavorably results in an increase in manufacturing cost. Further, with only such means as described in JP-A No. 2005-015857 (see, paragraph 0022) and JP-A No. 2002-273598 (see, paragraph 0020), the improvement of the brazability is insufficient.